In an optical network, to dynamically switch optical signals or optical packet signals, it is necessary that an optical packet signal or optical signal frame having a data added with a header or a label showing route information such as its destination is output onto the optical network, and an optical switcher sets a route for the optical packet signal according to the header. This method is theoretically the same with ordinary electric signal data transmission.
However, in optical transmission, a bit stream is usually as fast as 10 Gb/s or more, and accordingly it is a problem how to add a header containing destination information etc. to its data. To solve the problem, two configurations have been well known. One configuration is to arrange the destination information etc. as a header immediately before the data in a similar way to a configuration for an electric signal, and the other one is to add route information containing the destination information etc. as a label on an envelope of an optical signal using intensity modulation. As an example of the latter configuration, to put it concretely, a subcarrier superimposing system is well known in which a carrier of any frequency is modulated by the route information and intensity of an optical signal is modulated by the obtained modulation signal.
However, such a system having the above configuration in which the header is added directly before the data becomes inevitably high-priced because a high speed logic circuit is required to receive and decode the header.
In the subcarrier superimposing system, it is necessary to increase a degree of the intensity modulation to improve the detectivity of the label, and this causes a quality deterioration of the optical signal. In addition, a label signal cannot pass through a data regenerator or a wavelength converter which reforms a waveform of the optical signal.